The failure analysis using transmission electron microscopy (TEM) has been actively preceded in semiconductor industry. But due to the overlap issue and structural complexity of devices, it has become harder and harder to perform failure analysis using normal projected bright field (BF) and high angle annular dark field (HAADF) TEM images. To overcome these problems, 3-dimensional (3D) tomography technique has been suggested. In this work, we clarify the root cause of dark voltage contrast (DVC) failure at the bottom electrode contact region in PCRAM by using 3D tomography analysis. The 3D tomography samples were prepared in lamella shape by using focused ion beam (FIB). The electron energy loss spectroscopy (EELS) and 3D tomography analysis in scanning transmission electron microscope (STEM) HAADF mode were carried out. Through 3D tomography image reconstruction by AMIRA program, we observed ‘open contact fail’ between the BEC (Bottom Electrode Contact)-1 and the BEC (Bottom Electrode Contact)-2 at DVC region that could not be shown in 2D image.